MRI apparatuses are imaging apparatuses for imaging an arbitrary section of an imaging subject using the nuclear magnetic resonance phenomenon. Specifically, MRI apparatuses irradiate a radio frequency magnetic field on an imaging subject placed in a spatially uniform magnetic field (static magnetic field) to cause magnetic resonance, detect generated magnetic resonance signals, and perform image processing for the detected signals to obtain a tomographic image.
A device that irradiates a radio frequency magnetic field on an imaging subject and detects magnetic resonance signals generated from the imaging subject is called RF coil. The RF coil has a loop part (coil loop) for performing the irradiation and detection. If this coil loop is made smaller, sensitivity becomes higher, although sensitivity area becomes narrower. On the other hand, if the coil loop is made larger, the sensitivity area can be expanded. In this way, magnitude of sensitivity and size of sensitivity area of RF coil are in a trade-off relationship. Since magnetic resonance signals are signals of a rotating magnetic field generated in a direction perpendicular to the static magnetic field, it is preferred that the RF coil is disposed in such a direction that the RF coil can irradiate a magnetic field and detect signals in a direction perpendicular to the static magnetic field.
As described above, a smaller RF coil provides higher sensitivity, but shows narrower sensitivity area. As a means for solving this problem, there is a multi-channel array coil comprising a plurality of RF coils arranged in the form of array (refer to, for example, Non-patent document 1). Since multi-channel array coils show high sensitivity and wide sensitivity area, they constitute the current main stream of receiving RF coil. Each individual RF coil constituting a multi-channel array coil is henceforth referred to as subcoil.
If RF coils having the same resonance characteristics are disposed closely to each other, they usually interfere with each other by magnetic coupling. Since magnetic coupling degrades performances of the RF coils, it is essential to eliminate magnetic coupling between subcoils in a multi-channel array coil. Non-patent document 1 describes that magnetic coupling is reduced as far as possible by disposing the subcoils so that adjacent subcoils partially overlap with each other. Further, interference from subcoils other than the overlapping subcoils is reduced by using low input preamplifier, inductor, and capacitor so that a part of each coil loop has high impedance.
In recent years, high-speed imaging using difference of spatial sensitivities of individual subcoils of a multi-channel array coil (for example, refer to Non-patent document 2) is spreading. As for the high-speed imaging, a larger number of channels realize a higher imaging speed. Therefore, channel number of multi-channel array coil becomes further larger in recent years, and supermulti-channel array coils such as those of 32 channels or 128 channels are spreading at the present.